You ever hear of the phrase, "You catch more flies with honey than with vinegar?".

You are asking for free support for a free e-mail program from a Community Support Forum in which everyone is a volunteer. Please be nice.

Both Thunderbird and Firefox are free programs available from Mozilla. Of course both are supported by each other. That is not the issue in your case, though.

You need to tell Windows that Thunderbird is the default e-mail program and that Firefox is the default browser. Your problem appears to be the lack of the latter. Even though this is the Thunderbird support forum, I will help you with your Windows/Firefox problem.

You are using Windows 10, so: Click on Cortana search (the icon just to the right of the Start Menu button), and type "default". Click on the search result "Default Programs". See my first screenshot below.

A window will come up where you can "Choose default apps". Click on the program in the category you want, and then change it to what you want. Like Thunderbird for Email, and Firefox for Web browser.

To make sure all defaults for these programs are set in Windows: Scroll down to the bottom of this window and click on "Set defaults by app".

In the "Set Default Programs" window that comes up, scroll down to Thunderbird, select it, then click "Set this program as default" on the bottom, right side. See my second screenshot below.

Do the same for Firefox.

Done.

Hi Bruce,

 Yeah I know a lot to expect for free.  However I expected Mozilla to at least support it's own products.

 My Win 10 default is set to Firefox and it works except in Thunderbird.  

 I see Thunderbird has an option for setting a default browser, attached, but Firefox is not an option.

Best regards. Ray

Your attachment shows the setting for a default Search Engine, not browser.

I just realized: You never stated the exact problem you are encountering. You just imply that Thunderbird doesn't support Firefox.

So I'm not going to assume anything.

Please state what exactly you are doing, what the result is, and what it is that you expect to happen instead.

The attachment is for the Thunderbird search engine options, which looks like it doesn't support Firefox.

The problem is when I select text in the Thunderbird dialog and search it. It uses the search engines from the attached list above. I need Firefox as the default since others give undesirable results.

Now that we have gone through this exercise I'm wondering if Thunderbird can support Firefox since its not on the list.

OK, you are making me guess/assume things because you still haven't said exactly what you are doing, what happens, and what you want to happen differently.

I'm guessing that you select a word in an e-mail you see in Thunderbird, right-click it, and select the third option in the pop up menu which is Search XXXX for "search term"

Then you don't like what happens next.

The search results don't come up in the Firefox browser? Or you don't like the search results that do come up in the Firefox browser?

Firefox is a web browser, not a website that can be searched, like Google, Bing, Wikipedia, etc, which is set in the screenshot you showed.

There are various websites you can search, and different web browsers you can use to browse showing those various websites that you can search.

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A wild guess: You want to do this search with Google?

You can add Google to the Thunderbird search engines with the Thunderbird add-on "Google Search for Thunderbird":

Thunderbird menu: Tools: Add-ons: Enter "Google Search" in the search field in the top right and press the Enter key on your keyboard. In the search results, find "Google Search for Thunderbird" and press the "Install" button beside it. It will prompt you to restart Thunderbird. Do so. Go back to the Options page you have in your screenshot and make sure that the search engine is "Google".

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If none of this helps you, then I implore again:

What is it exactly you are doing, what are the results, and what are the results you want instead.

If you don't give this information, I cannot help any further.

Doesn't answer my question on how to make FireFox my iMAc's default browser.

That is a function of your operating system. Google how to set default programs on a MAC. This is not a Thunderbird or Firefox problem.